Static Metamorphism
Some of the changes occur because the rocks are at great depths. As more and more younger rocks are deposited on top of them, the older rocks become deeply buried. The great thicknesses of younger rocks are heavy, and they squeeze and press down on the rocks beneath them. The deeply buried rocks are also hotter than surface rocks. In general, the temperature increases about 1° Fahrenheit for each 50 feet of depth below the surface. The change of deeply buried rocks into new rocks by pressure and heat is known as static metamorphism.
Contact Metamorphism
Another method of change or metamorphism involves molten [igneous] rock material. When hot [magma] moves up through rocks, it not only heats and pushes them, but it also may soak them with liquids and gases, causing the nearby rocks to change into new rocks, by a process called contact metamorphism.
Some rocks are altered by heat and [fluids] when they are invaded by hot [magma] in a process called contact metamorphism.
UNALTERED ROCK [METAMORPHIC ROCK] [MAGMA]
Dynamic Metamorphism
Still another rock-changing process is one that is associated with mountain building. When mountains are formed, heat and great pressures develop deep within the earth’s crust. The flat layers of rock are then slowly pushed and squeezed so that they bend up into arches, [fracture], or slide over each other. These forces cause great changes in the rocks in widespread areas. This process of change is known as dynamic metamorphism.